The present invention relates to antennas for radio communications, and in particular although not exclusively to multiple input multiple output (MIMO) radio communications systems.
The demand for wireless communication systems has grown steadily over recent decades, and has included several technological jumps over this time, particularly in the area of cellular and wireless local area network (WLAN) communication systems Analogue cellular phones have been replaced with digital handsets using for example GSM and CDMA technologies, and so called third generation systems such as UMTS are now being introduced. Similarly WLAN technologies such as HyperLan and IEEE 802.11b are also being introduced. The number of users continues to increase and data traffic is now becoming an important part of the wireless network. Both of these factors mean that it is important for operators to look for methods of increasing the capacity of their networks to meet future demands.
A relatively new radio communications technology known as multiple input multiple output (MIMO) systems which provide for increased system capacity by exploiting orthogonal propagation paths are being investigated for use in cellular and WLAN applications. Such systems exploit parallel or decorrelated radio channels between a receive and transmit antenna. A number of antenna elements are used on both the transmitter and receiver antennas which together with appropriate beam forming and signal processing technologies are capable of providing two or more orthogonal radio propagation channels between the two antennas. The antenna elements are spaced apart in order to decorrelate the signals associated with adjacent antenna elements. A problem exists, particularly at the base station of cellular systems where there is a low angle spread which requires the spacings to be very large, of the order of 10-20 wavelengths. This is typically less of a problem at the terminal of cellular systems there there is usually a large angle spread due to a large number of reflecting objects close to the terminal so that much smaller spacings between the antenna elements (between 0.25 and 0.5 wavelengths) are adequate. However this is still problematic given the increasing miniaturisation of these terminals. In addition the orthogonatity of the propagation paths is sensitive to the orientation of the terminal antenna which is especially problematic in portable terminals. Similar problems exist in WLAN applications.
There is a need for improved antenna arrangements for use with MIMO systems, particularly at the cellular and WLAN terminals.
It is an object of the present invention to provide an improved or at least alternative antenna arrangement for use with MIMO communications systems. It is a further object of the present invention to provide a MIMO antenna having a performance largely independent of antenna orientation.
In accordance with a first aspect the present invention provides an antenna arrangement comprising: a first, a second, and a third ground plane each arranged substantially perpendicular to the others; a first, a second, and a third antenna element associated with respectively said first, said second, and said third ground planes.
Preferably an antenna arrangement wherein each said antenna element is a transmission line loaded antenna having a section extending perpendicular and a section extending in parallel with respect to said associated ground plane.
Preferably an antenna arrangement wherein said antenna elements are arranged such that at any azimuth angle one of said antenna elements has a first predetermined polarisation, and another of said antenna elements has an orthogonal polarisation.
Preferably an antenna arrangement wherein said antenna elements are arranged such that a third of said antenna elements has one of said polarisations and is spaced apart from the antenna element having the same polarisation in any azimuth angle.
Preferably an antenna arrangement as further comprising a fourth antenna element associated with a fourth ground plane, wherein said fourth ground plane is parallel with said first ground plane, and wherein the parallel section of said first antenna element extends in a direction perpendicular to that of the parallel section of said fourth antenna element.
Preferably an antenna arrangement wherein said first and fourth ground planes are co-planar.
Preferably an antenna arrangement, wherein said antenna elements are planar inverted F antennas.
Preferably an antenna arrangement and further comprising an additional antenna element associated with each said ground plane.
Preferably an antenna arrangement wherein each said additional antenna element has a parallel section extending in a direction perpendicular to that of the parallel section of the other antenna on the associated ground plane.
Preferably an antenna arrangement as claimed in claim 1 wherein a said ground plane comprises metallised portions removed therefrom in order to improve the azimuth radiation pattern.
In accordance with a second aspect the present invention provides a Multiple input Multiple Output communications system comprising: an antenna arrangement comprising: a first, a second, and a third ground plane each arranged substantially perpendicular to the others; a first, a second, and a third antenna element associated with respectively said first, said second, and said third ground planes; and signal processing means arranged to implement a Space Time Coding communications transmission or reception method.
In accordance with a third aspect the present invention provides a method of implementing a wireless communications system and comprising: providing a first, a second, and a third ground plane each arranged substantially perpendicular to the others; a first, a second, and a third antenna element associated with respectively said first, said second, and said third ground planes; and providing signal processing means arranged to implement a Space Time Coding communications transmission or reception method.
In accordance with a further aspect the present invention provides an antenna arrangement comprising: a first, a second, and a third ground plane each arranged substantially perpendicular to the others; a first, a second, and a third antenna element associated with respectively said first, said second, and said third ground planes, each antenna element arranged to radiate in a predetermined polarisation in elevation and azimuth with respect to said respective associated ground plane.